Invasive species take a toll on their surroundings. But that doesn’t mean that invaders are universally destructive. Ecosystems are dynamic, and once an invasive species arrives, it can develop intricate relationships with other organisms. Sometimes, an invader becomes an integral link in a delicate ecological web, complicating efforts to eradicate it.
One example lives in the salt marshes of Northern California. An aggressive species of cordgrass called Spartina alterniflora arrived in California in the 1970s and has taken hold in San Francisco Bay, hybridizing with a native species, Spartina foliosa. The invasive hybrid grows quickly, displacing native species and choking precious waterways. But it has also become a vital nesting and foraging habitat for the endangered California clapper rail. As soon as land managers began efforts to eradicate it, the local clapper rail population plummeted by nearly half.
It’s quite an ecological conundrum: eradicating the invasive Spartina hybrid puts the vulnerable clapper rail at risk, while leaving the invader alone would result in further damage to the marshes.
In a new paper in Science, a group of researchers modeled the outcomes of various eradication plans in an attempt to maximize effectiveness and minimize ecological side effects and financial cost. They found that the ideal plan strikes a delicate balance between getting rid of the Spartina hybrid and helping native Spartina recolonize the area.
The key is to take it slow. First, land managers should work to cull the invasive Spartina until just enough remains to support the clapper rail population. At that point, eradication must be put on hold, and the work should shift to replanting the native Spartina. Since the native species grows much more slowly than its invasive cousin, several years may pass before it is robust enough to support the clapper rails’ needs. Only then can the eradication efforts begin again, this time to get rid of the invasive species completely.
Clearly, this three-step process will make much slower progress than a plan where all efforts are concentrated on a simple eradication. But it’s much more effective on an ecosystem-wide level, since it takes into account the needs of the clapper rail, as well as any other species that have come to depend on the invasive Spartina.
Ideally, this process will serve as a model for other habitats where threatened species rely on invasive species. One of these situations is playing out in the southwestern US, where endangered southwestern willow flycatchers are nesting in the invasive tamarisk trees, which have crowded out native vegetation. The effort to eradicate tamarisk has come to a near-standstill, thanks to its importance to the flycatchers. But it’s likely that land managers can put some of the new paper’s principles to use in gradually restoring the ecosystem to a state that supports the birds.
It’s difficult to anticipate what will happen to an ecosystem when a species is removed, and this uncertainty makes eradication a complex issue. But the Spartina-clapper rail problem—and the paper’s potential solution—demonstrates the importance of thinking and acting on a larger scale when it comes to invasive species. In most cases, the overarching goal of conservation efforts should be a healthy and well-functioning ecosystem, rather than the complete and immediate eradication of an invader.